1. Field of the Invention
This invention relates generally to a back printing line printer, and more particularly to a line printer that includes a bank of print hammers, the face of each hammer spanning three print positions, and a recirculating character belt, the pitch between adjacent characters being greater than the width of a hammer face such that each hammer is able to strike a single selected character in three successive print positions.
2. Description of the Prior Art
In many printers in the prior art, a chain of uniformly spaced print characters is moved at constant speed in a continuous path, a portion of the path lying on a print line extending transversely of a paper-receiving web, which in turn is fed intermittently, line by line, into registry with the print line by a feed mechanism under electronic control. A plurality of electromagnetic hammers located respectively on print columns in the print line, are operated selectively by controlled electronic firing circuits to impress the web against a selected print character on the print line as that print character moves into registry with the print column on which the selected hammer is located. Accordingly, in these printers, each hammer spans a single print position such that 132 hammers are required to cover a 132 character print line.
In order to decrease the manufacturing and maintenance costs of a printer, attempts have been made to reduce the number of hammers included within the hammer bank and their respective electronic driving circuits. In one instance the hammers are separated by two print positions and the paper is shuttled laterally over the print positions. However, the complexity of shuttling the paper precludes this attempt from viability in the market place. In another type of printer, one-half of the number of hammers is utilized, and these hammers are mechanically shifted such that each hammer prints odd and even characters. Alternatively, one-fourth the number of hammers is used and shifted mechanically such that each hammer is only able to print every fourth character. Still another approach is to utilize one-half the number of hammers and to separate the hammers such that the pitch between adjacent hammers spans two print positions. In conjunction with this hammer configuration, a character belt with a pitch of two print positions is used. Thus, at any one time only one character is in front of each hammer and hence, two passes of the character set is required to print a full line. Also, in each of these configurations, because of the movement required of either the hammer or the paper, or because of high character velocity, poor print quality, higher product cost and low machine reliability results.